WO2022108475A1 - METHOD FOR PRODUCING A CORELESS β-SILICON CARBIDE FIBER - Google Patents

Abstract

The present invention relates to a method for producing a coreless β-silicon carbide fiber. The method includes four steps. In the first step, a multifilament polymer fiber is formed from a polycarbosilane melt by extrusion. In the second step, thermal-oxidative cross-linking is carried out, in the third step the cured polymer fiber thus produced is carbonized and converted into a ceramic phase, and in the fourth step the β-silicon carbide fiber thus produced is finished. As a result, fibers are produced having enhanced strength characteristics, greater resistance to high temperatures, high creep resistance, stable fiber properties, and an optimal average fiber diameter.

Description

Method for producing coreless silicon carbide fiber of P-modification

[0001] The invention relates to the field of the chemical industry, namely to a method for producing a p-modification (P-SiC) coreless silicon carbide fiber. Coreless silicon carbide fibers can be used in nuclear power, aerospace engineering, and other high-tech industries.

[0002] A method for producing textile silicon carbide materials is known from the prior art (RF patent No. 2694340, IPC С01В 32/956, С01В 32/97, published on 07/11/2019) by siliconizing heat treatment of carbon fiber precursors in a SiO gas environment. Silicifying heat treatment of carbon fiber precursors is carried out in a batch reactor, inside which is a material that generates SiO gas when heated; heat treatment is carried out under conditions of continuous vacuum pumping of gaseous products at a temperature of 1350-1650°C with exposure for 60-600 minutes. The rate of temperature rise during heating in the range from 1300°C to the siliconizing heat treatment temperature is 0.2-6.0°C/min.

[0003] A disadvantage of the known invention is the instability of fiber properties due to non-stoichiometric multi-phase composition.

[0004] A method and device for continuous production of high-quality crystalline silicon carbide, in particular, in the form of nanocrystalline fibers, is known from the prior art (US patent US2018002829, IPC C23C 16/32, SZOV .25/00, SZOV 29/36, SZOV 29/ 66, published 01/04/2018). The method includes the steps: a) introducing into the reactor a mixture of precursors with a source of silicon, a source of carbon and, optionally, a doping agent; b) subjecting said mixture of precursors to a gasification temperature in the reactor; c) applying crystalline silicon carbide to the substrate by controlling the crystallization temperature on the substrate; d) moving the substrate to the scraper; and e) transferring the produced silicon carbide to a collection container by removing the produced silicon carbide from the substrate by moving the substrate along the scraper. [0005] The disadvantages of the known invention are the instability of fiber properties due to the non-stoichiometric multi-phase composition of the resulting crystalline silicon carbide, as well as the impossibility of obtaining continuous coreless SiC fibers under these manufacturing conditions.

[0006] In the prior art, a spinning solution for electrospinning, a method for producing fibers by electrospinning and silicon carbide fibers (RF patent No. 2427673, IPC D01D 1/02, D01D 5/00, published 08/27/2011). The invention relates to a technology for producing silicon carbide fibers. The spinning solution for the electrospinning of a polymer precursor of silicon carbide fibers contains a 50-70% solution of polycarbosilane with an average molecular weight of 800-1500 a.m.u., a crosslinking agent and a photoinitiator in the following molar ratio of components: polycarbosilane/crosslinking agent/photoinitiator = 1/( 0.5-1.5)/(0.5-2). The method for producing silicon carbide fibers includes preparing a spinning solution, electrospinning silicon carbide precursor fibers from the spinning solution with simultaneous crosslinking of the precursor fibers by irradiation with light in the visible or ultraviolet radiation range, and heat treatment of the precursor fibers to convert them into silicon carbide fibers. Silicon carbide fibers made according to this process have an average diameter of 50 nm to 2 µm and a porosity of less than 10 m ² /g.

[0007] A disadvantage of the known invention is the small average diameter of the silicon carbide fibers produced by this method, as well as the impossibility of obtaining long-length continuous coreless SiC fibers under these manufacturing conditions.

[0008] Stoichiometric silicon carbide fibers from thermochemically cured polysilazanes are known from the prior art (US patent US2011212329, IPC COIB 31/36, published 09/01/2011). Polycrystalline stoichiometric thin SiC fiber, practically free of impurities, is obtained using a ceramic polymer precursor. The ceramic polymer is produced by reacting a mixture of chlorodisilane, boron trichloride, and vinylchlorodisilane with an excess of hexamethyldisilazane to form a ceramic polymer resin, which can then be spun, cured, pyrolyzed, and heat treated to form a finished SiC fiber.

[0009] The disadvantages of the known invention are the insufficient strength characteristics of the silicon carbide fiber, as well as the complex procedure for preparing the used carbon fiber precursor. [0010] The closest analogue, taken as a prototype, is a method for producing silicon carbide fiber (Japanese patent JP2019137935, IPC C01B 32/977, D01F 9/10, published on 08/22/2019), including: a stage for obtaining polycarbosilane, a dry spinning stage for obtaining polycarbosilane in fiber form; and a firing step for producing silicon carbide fiber from the polycarbosilane fiber. The resulting silicon carbide fiber is inexpensive and has a diameter of 10 µm or less.

[OOP] The disadvantage of the prototype is the presence of impurity hydrogen in the composition of the fiber due to the participation of gaseous hydrogen in the process of carbonization of the fiber.

[0012] The technical result of the invention is the possibility of obtaining continuous coreless silicon carbide fibers, increasing their strength characteristics, increasing resistance to high temperatures and high creep resistance, stability of fiber properties, optimal average fiber diameter, simplifying the procedure for preparing the used carbon fiber precursor, the absence of impurities in the composition fibers.

[0013] The method for producing coreless silicon carbide fiber P-modification, according to the present invention, includes the successive stages of spinning a fiber from a melt of a fiber-forming organosilicon polymer, thermochemical oxidative crosslinking, carbonization and sizing of the fiber.

[0014] The invention is illustrated in the following drawings.

[0015] FIG. 1 shows a plot of polycarbosilane (PCS) fiber weight change under various thermal-oxidative crosslinking conditions.

[0016] FIG. 2 shows a typical PCC fiber carbonization program.

[0017] FIG. 3 is a table showing the temperature characteristics of PCBs determined prior to the molding process.

[0018] FIG. 4 shows an extrusion-forming unit for forming a multifilament polymer fiber from PCS, where 1 is a loading hopper; 2 - material cylinder; 3 - auger; 4 - clutch; 5 - extruder drive; 6 - heating elements; 7 - gear pump; 8 - gear pump drive; 9 - spinneret set; 10 - under-die furnace; 11 - fiber winding device; 12 - vacuum pump.

[0019] FIG. 5 shows a diagram of a spinneret set, where 13 is a glass (body); 14 - die; 15 - perforated disc; 16 - melt filter; 17 - connecting sleeve; 18 - nut; 19 - gasket.

[0020] FIG. 6 shows a photograph of PCS granules. [0021] In FIG. 7 is a photograph of a receiver including a fiber winder and a fiber spreader.

[0022] FIG. 8 and 9 show photographs of PCS polymer fiber spools.

[0023] FIG. 10 is a table showing the main parameters of fiber extrusion on the extrusion molding unit.

[0024] FIG. 11 shows a photograph of a graphite creel.

[0025] FIG. 12 shows a photograph of fibers prepared for carbonization.

[0026] FIG. 13 shows a thermogravimetric analysis (TGA) of an oxidized fiber.

[0027] In FIG. 14-19 show photographs of the appearance of 0-SiC fiber.

[0028] In FIG. 20 shows a schematic diagram of applying a sizing to a 0-SiC fiber, where 20 are cups with a sizing and coils of 0-SiC fibers; 21 - hook; 22 - oven; 23 - roller; 24 - winding device; 25 - finished thread.

[0029] FIG. 21 shows a photograph of a cup with a sizing and a coil of 0-SiC fibers.

[0030] FIG. 22 and 23 show the surface of the 0-SiC fiber (x500 magnification).

[0031] In FIG. 24-31 show a scanning electron microscopy (SEM) micrograph of a 0-SiC fiber.

[0032] FIG. 32 is a table showing the characteristics of a pilot batch of 0-SiC fiber.

[0033] FIG. 33 and 34 show a pilot batch of 0-modification coreless silicon carbide fiber.

[0034] Obtaining refractory coreless silicon carbide fibers consists in spinning a fiber from a melt of a fiber-forming organosilicon polymer, followed by its curing, pyrolysis and obtaining a SiC filament.

[0035] A key feature of coreless SiC fibers is their ultrafine microstructure, which imparts high tensile strength. Their creep and thermal resistance are mainly determined by the stoichiometry and, in particular, by the oxygen content. Therefore, oxygen-reduced coreless silicon carbide fibers have better high temperature resistance and higher creep resistance.

[0036] The essence of the invention lies in the fact that in order to obtain a coreless silicon carbide fiber of 0-modification, a multifilament polymer fiber is formed by extrusion from a melt of a fiber-forming organosilicon polymer, thermal oxidative crosslinking is carried out, carbonization of the obtained cured polymer fiber to obtain coreless silicon carbide fiber P-modification, and sizing of silicon carbide fiber -modification.

[0037] Polycarbosilane (PCS) with methyl substituents and methylene bridges - poly(oligo)dimethylsilylenemethylenes - was chosen as a fiber-forming organosilicon polymer, since after their pyrolysis, silicon carbide closest to the stoichiometric composition is formed. Most of the excess carbon from the side methyl groups is removed during the pyrolysis process and does not enter the ceramic.

[0038] A continuous and stable process for producing coreless ceramic fibers requires that the organosilicon polymer have a relative (number average) molecular weight of -1000^-2500 and suitable viscoelastic properties to ensure stable spinning and fiber drawing without breakage.

[0039] A suitable raw material for the synthesis of PCS is polydimethylsilane, since it has an acceptable stoichiometric ratio of Si: C (2: 1), is affordable from an economic point of view, does not contain oxygen, which adversely affects the properties of the final product, and the decomposition reaction proceeds relatively quickly without catalysts.

[0040] The first stage of the method is the implementation of the formation of an amorphous multifilament polymer fiber by melt extrusion of a fiber-forming organosilicon polymer, which is selected as polycarbosilane.

[0041] Extrusion is a method and process for producing products from polymeric materials by forcing a material melt through a forming hole - a die. The equipment for this method is an extruder. The main function of the extruder is to provide enough pressure in the material to force the material through the die.

[0042] An extrusion forming unit for forming a coreless silicon carbide continuous fiber operates in accordance with the following methodology:

1. The polymeric material is loaded in the form of pieces, granules or chips into the hermetic loading hopper of the extruder, with the possibility of vacuuming and purging with an inert gas;

2. The polymer material is fed from the extruder feed hopper to the polymer melt preparation unit, heated to the selected temperature, but not more than 400 °C (in this case, to select the temperature, the optimal molding temperature is determined);

3. The prepared polymer melt is fed to a hermetic heated fiber forming unit, in which an extrusion gear (gear) pump with an adjustable speed, driven by an electric motor, continuously pumps the melted polymer onto the extruder head (spinneret unit) at a certain speed and pressure, while the polymer melt, being distributed over the entire surface area of the die, is forced through all the holes at the same speed, which makes it possible to obtain a bundle of continuous fibers of the required length with the given parameters in terms of the number of filaments and the diameter of each filament;

4. Gradual cooling is carried out, followed by curing of the resulting bundle of fibers in an inert atmosphere after leaving the spunbond assembly - first in a heated sub-spinnery space, then with cooling in an inert gas medium to room temperature;

5. The cooled fiber bundle is received at the exit from the cooling shaft with its winding on spools.

[0043] The optimal spinning temperature (or the so-called T _c.n. - the temperature of a good thread) is selected from considerations of viscosity and temperature of degradation of the polymer. For each specific sample, this temperature is slightly different. The choice of the optimal molding temperature is carried out on the basis of the characteristic temperatures of the polymer: the temperature is selected from the interval above the fiberization start temperature (T ₂ ) and below the dropping point (Tz). The tests carried out show that the optimal molding temperature is on average 30-40°C higher than the fiberization start temperature (T ₂ ). It can be noted that at a lower viscosity, the moldability of the polymer is improved.

[0044] Reception of the formed polymer fiber from the extrusion-forming unit is carried out on a special receiving device, which includes a fiber winding device and a fiber layout device.

[0045] The speed of the spreader in the fiber spreader allows you to change the angle of winding the fiber on the spool, that is, when the speed decreases and the winding angle decreases, parallel winding occurs, when the speed increases and the winding angle increases, cross winding occurs. [0046] The winding speed in the fiber winder can be varied in the range of 1 to 1500 rpm, while in the present invention, the winding speed used varies in the range of 400-800 rpm. It directly influences the under-spinner drawing of the fiber and promotes the orientational drawing of PCS macromolecules. During the spinning process, spinning of the fiber occurs, due to the fact that the acceptance rate is higher than the feed rate of the molten polymer from the spinneret.

[0047] Thus, for the implementation of the first stage of the method, each batch of polycarbosilane is preliminarily determined by the temperature of the start of fiber formation and the dropping point, then polycarbosilane is introduced into the extruder at room temperature (in the form of pieces, granules or chips with a size of 2-7 mm) and the polycarbosilane is heated to the optimal molding temperature selected from the range above the fiberization start temperature and below the dropping point, to obtain a homogeneous polycarbosilane melt. Polycarbosilane melt is squeezed out through a die at a pressure of 1.5-4.0 MPa to form a polymer fiber. The formed polymer fiber is gradually cooled in an inert atmosphere to room temperature, followed by curing. The polymer fiber is received by a fiber winding device having a winding speed greater than the polymer fiber feed rate. As a result, a formed polymer fiber is formed. As an inert atmosphere, nitrogen or argon gas can be used, for example.

[0048] It should be noted that the spinning process is an important step towards obtaining high-quality polymer fibers, since the defects formed during spinning (sagging, gas inclusions) subsequently pass into the finished ceramic fiber, which largely negatively affects its strength . The spinning parameters selected according to the present invention make it possible to obtain an optimal melt flow - without the presence of gas formations (bubbles) in the polymer fiber, which will either remain in the fiber or lead to breakage; without extrudate sticking to the outer surface of the die.

[0049] The second stage of the method is the implementation of thermal-oxidative crosslinking - curing (oxidation) in air at an elevated temperature according to the following scheme: sn _h 1 sn 1 ¹¹ O oO sl - - n 1 t JC - Si - C - Si - + 3 C> 2 - * ■ " - Si - C - Si - + 2n + n CO ₂ to 1 H, 1 sn ₃ sn ₃ s ¹ n ₃ ^H 2 2 A _ p n ° S ~ t

[0050] Cross-linking of n 1 ~ o 11l _ imer molecules occurs due to the oxidation of the Si-H and Si-CH3 groups and the formation of new Si-O-Si and Si-0-C bonds. At the same time, the mass of fibers increases by an average of 6-15%.

[0051] In studies to select the optimal package parameters, graphite foil, as well as individual fibers, were cured in an oxidation oven at temperatures up to 175-300 ° C with a heating rate of 3-10 ° C / h.

[0052] After examining the samples, it was determined that the samples received a different elemental composition due to different oxidation regimes. In addition, according to the change in the mass of fibers, i.e. weight gain under different conditions, one can judge the most effective temperature at which thermal-oxidative crosslinking occurs. From FIG. It can be seen from Table 1 that these are temperatures up to 250 °C, since then a decrease in mass occurs, and as a result, degradation begins with the removal of low molecular weight products.

[0053] Thus, it is possible to judge the optimal temperature of thermal-oxidative crosslinking, selected from the range of 175-250 °C. The optimal heating rate is 3-10 °C/h. The resulting spun polymer fibers wound on graphite packages are cured in an oxidation oven at a selected temperature to increase the weight by 6-15%.

[0054] The third stage of the method is the implementation of the carbonization of the obtained cured polymer fiber with its translation into the ceramic phase.

[0055] To do this, the cured polymer fiber is processed in a high-temperature furnace in an inert atmosphere or in a vacuum with stepped heating to a temperature of 1100-1300 °C at a heating rate of 2.5-5 °C/min. As an inert atmosphere, nitrogen and/or argon gas can be used, for example.

[0056] The reaction proceeds according to the scheme:

N ₂ , AG 1 1 1 + 0.2p H ₂ o+ 0.9p CO ₂ + o.bp CH ₄ + 2.6n H ₂ + o.5n C ~GGG p p

[0057] At the same time, a mandatory exposure is carried out for 30 minutes at temperatures of 400, 700 and 1250 ° C, since these temperatures correspond to intensive removal from material of low molecular weight substances. The developed PCS fiber carbonization program is shown in Fig. 2.

[0058] Thus, the staged heating program for the carbonization of PCS fibers in a high-temperature vacuum furnace in accordance with the above requirements includes the following stages:

1. Vacuuming the oven (up to 100 Pa);

2. Filling the furnace space with an inert gas (if necessary);

3. Heating up to 400 °C at a heating rate of 2.5-5 °C/min;

4. Exposure 30 minutes at 400 °C;

5. Heating up to 700 °C at a heating rate of 2.5-5 °C/min;

6. Exposure 30 minutes at 700 °C;

7. Heating up to 1250 °C at a heating rate of 2.5-5 °C/min;

8. Exposure 30 minutes at 1250 °C;

9. Furnace cooling.

[0059] The total process time, excluding oven cooling, is about 7 hours. This program is typical, it is suitable for work in vacuum and in an inert environment.

[0060] The high temperature furnace is then cooled down.

[0061] The fourth step of the method is the sizing of the resulting 0-modification silicon carbide fiber.

[0062] The purpose of sizing is to improve the technological properties of threads and tows: facilitating processing processes (for example, weaving), protecting brittle fibers from mechanical damage during processing, and reducing electrification. The sizing composition, as a rule, is an emulsion or suspension consisting of polymeric components - emulsifiers, film-forming, antistatic and connecting agents that help level and protect the fiber, improve its wettability.

[0063] As a coupling agent, in particular, a solution of polyvinyl alcohol with distilled water can be selected. Polyvinyl alcohol is an excellent emulsifying, adhesive and film forming polymer. It has high tensile strength and flexibility.

[0064] When selecting the concentration of a solution of polyvinyl alcohol with distilled water, the following solution concentrations were used: 0.25%, 0.5%, 1% and 3%. AT As a result of the experiments, it was found that the higher the concentration of the solution, the more rigid and brittle the finished thread is. Therefore, the minimum concentration (0.25%) was chosen, which was optimal for applying the sizing composition to the fibers (3-SiC.

[0065] Thus, one of the sizing options is to place the silicon carbide fiber in a container with a solution of polyvinyl alcohol at a concentration of 0.25%, followed by drying until a constant weight is reached.

[0066] Immediately prior to use of the P-modification coreless silicon carbide fiber, the sizing should be washed off. The advantage of the selected sizing is the ease of its removal from the fiber surface without the use of organic solvents. Namely, to remove the sizing, P-modification coreless silicon carbide fiber is washed in warm water at a temperature of 35–40 °C and dried at a temperature of 600 °C for 30 min in air.

[0067] The choice of the specified sizing does not limit the present invention, but is one of the possible options.

[0068] Examples of the method for producing P-modification coreless silicon carbide fiber.

[0069] Example 1.

[0070] The formation of a multifilament polymer fiber, which is the first step in the process for producing a P-modification coreless silicon carbide fiber, was carried out by polycarbosilane (PCS) extrusion.

[0071] Preliminary, the optimal processing temperatures were determined for the PKC sample. As a result, the following data were obtained: softening temperature (T[) 200°C, fiberization start temperature (T ₂ ) 220°C and dropping point (Tz) 280°C. The temperature of a good thread T _{x n} . (optimum molding temperature) was chosen 30°C more than the fiberization start temperature - 250°C. In the table in Fig. 3 shows the temperature characteristics, sample No. 1.

[0072] Next, the extruder (extrusion-forming unit) was turned on in advance and the heating of the working parts was set up, taking into account the optimal molding temperature (T _{x n} .) 250°C.

[0073] A schematic diagram of an extruder for forming a multifilament polymer fiber from PCS is shown in FIG. 4. The extrusion-forming unit contains a loading hopper 1, a material cylinder 2, a screw 3, a coupling 4, a drive 5 extruder, heating elements 6, gear pump 7, gear pump drive 8, spunbond kit 9, spinneret oven 10, fiber winding device I, vacuum pump 12. Diagram of spunbond kit 9 is shown in Fig. 5. The spinneret kit 9 contains coaxial structural elements arranged in series - a cup (body) 13 in the form of a flange, a die 14, a perforated disk 15, a melt filter 16, a connecting sleeve 17, a nut 18, a gasket 19. The number of holes in the used spinneret 14 was 48 PCS. The working volume of the gear pump 7 is 0.6 cm ³ .

[0074] PCS was taken in the form of granules with a size of 2 - 7 mm (Fig. 6) and loaded into bin 1 (see Fig. 4), it was purged with an inert gas to prevent the interaction of PCS with atmospheric oxygen, while hopper 1 was evacuated using vacuum pump 12. [0075] The polymer material was fed from the loading hopper 1 to the polymer melt preparation unit, which was heated by heating elements 6 to a temperature of 250°C. The polymer melt preparation unit consists of a material cylinder 2 and a screw 3, which is driven through a clutch 4 by an electric drive of the extruder 5.

[0076] As a result of heating, the PCS granules were transformed into a PCS melt with a viscosity of 459.9 Pa-s.

[0077] Then, the prepared PCS melt was fed to a sealed heated fiber spinning unit, in which the gear pump 7 with an adjustable speed, driven by the gear pump drive motor 8, continuously at a speed of 6 rpm and a pressure of 5 MPa pumped the melted PCS onto spinneret set 9. At the same time, the PCS melt, being distributed over the entire surface area of the spinneret 14, was forced through all 48 holes at the same speed at a pressure of 3.00 MPa, which led to the formation of a bundle of continuous fibers.

[0078] Next, gradual cooling was carried out, followed by curing of the obtained bundle of fibers in an inert atmosphere - nitrogen gas. First, cooling occurs after exiting the spunbond set 9 in a heated under-spinning furnace 10 in an inert gas environment, then in an inert gas environment outside the under-spinning furnace 10 to room temperature, while the cooled formed polymer fiber was received at the fiber winding device 11.

[0079] The winding speed in the fiber winder is 440 rpm, which is a higher speed than the feed rate of the molten polymer fiber from the spinneret - 6 rpm, for under-die drawing of the fiber. [0080] The spreader speed in the fiber spreader, equal to 52 rpm, made it possible to change the angle of winding the fiber onto the spool. The receiving device, which includes a fiber winder and a fiber spreader, is shown in FIG. 7.

[0081] The PCS samples showed excellent formability, were drawn into a thin continuous filament, extrusion proceeded without pressure surges, and no gas inclusions were observed in the extrudate.

[0082] As a result, a batch of PCS polymer fibers was produced, which are brittle white fibers with a thickness of 25 microns. The produced batch of PCS fibers is shown in Fig. 8 and 9. After spinning, the fibers wrapped around the graphite packages were weighed on an analytical balance.

[0083] In the table in FIG. 10 shows the main parameters of fiber extrusion on the extrusion-forming unit, sample No. 1.

[0084] Next, the second stage of the method was carried out, namely, the polymer fiber was subjected to thermochemical crosslinking - curing (oxidation) in air at a temperature of 175 ° C with a heating rate of 3 ° C / h.

[0085] The crosslinking of polymer molecules occurred due to the oxidation of the Si-H and Si-CH3 groups and the formation of new Si-0-Si and Si-O-C bonds.

[0086] The fibers turned yellow, while their weight increased by an average of 6%.

[0087] After the oxidation process, the fibers wound on graphite packages were weighed on an analytical balance and installed on a special creel made of graphite. In FIG. 11 shows a graphite creel.

[0088] For the implementation of the third stage of the method, the cured PCS polymer fiber was transferred to the ceramic phase, that is, the sample was carbonized. To do this, it was subjected to processing in a high-temperature vacuum furnace with stepped heating up to 1100-1300 ° C in an inert atmosphere (nitrogen gases N2 and Ar).

[0089] The fiber creel was installed in the carbonization furnace as shown in FIG. 12, the oven was closed and evacuated. Next, the furnace was filled with an inert gas, and the PCS fiber carbonization process was carried out with stepped heating according to a program that included the following stages:

- Vacuuming the oven (up to 100 Pa);

- Filling the furnace space with an inert gas;

- Heating up to 400 °C at a heating rate of 4.4 °C/min; 2.5-5 °С/min

- Exposure at 400 °C (30 minutes); - Heating up to 700 °C at a heating rate of 2.5 °C/min; 2.5-5 °С/min

- Exposure at 700 °C (30 minutes);

- Heating up to 1250 °C at a heating rate of 4.5 °C/min; 2.5-5 °С/min

- Exposure at 1250 °C (30 minutes);

- Furnace cooling.

[0090] This program is shown in FIG. 2.

[0091] Oxidized fiber TGA data supporting the choice of program temperature parameters is shown in FIG. 13.

[0092] Photographs of the α-SiC fiber after the carbonization process are shown in FIG. 14-19.

[0093] The formed silicon carbide fibers [3 modifications had a thickness of 15-22 microns. After carbonization, the fibers acquired a black color, a shiny surface, significant shrinkage (up to 25%) became noticeable, color shades of blue and brown were observed.

[0094] The process of obtaining silicon carbide fibers in the fourth, final stage, includes sizing. A solution of polyvinyl alcohol with distilled water with a concentration of 0.25% was taken as a coupling agent.

[0095] The application of the sizing was carried out in accordance with the method shown in FIG. 20. A coil of [3-SiC] fibers without a coil was placed in glass 20 with a sizing, and a bundle was wound from the inside (from its center). The beam passed through the guide hook 21 into the oven 22 for drying the sizing. After exiting the furnace, the bundle went around roller 23, and the finished multifilament thread [3-SiC with a size of 25] was wound onto the winding device 24. It should be noted that the number of cups with coils of fibers and a size for simultaneous winding can reach five, depending on the required finished thread.

[0096] A sizing cup with a coil of P-SiC fibers is shown in FIG. 21.

[0097] This resulted in over 100 meters of continuous length of finished sized multifilament P-SiC.

[0098] The manufactured P-SiC fiber was sent for physical and chemical studies using optical microscopy, SEM, TGA, XRF (X-ray phase analysis), etc.

[0099] Microstructural and phase analysis of P-SiC fiber. The surface morphology of the P-SiC fiber was studied by optical microscopy using metallographic microvisor pVizo-MET-221, Lomo Photonics (Russia) with hardware magnification x200-x500.

[0100] The results obtained are shown in FIG. 22-23 showing the surface of the |3-SiC fiber (x500 magnification). The pictures show that the fibers are smooth, without defects and have a fine-grained structure.

[0101] In addition, the surface morphology of the 0-SiC fiber was studied by SEM. The data are presented in figures 24-31.

[0102] SEM micrographs showed that P-SiC ceramic fibers with a diameter of 15-22 μm have a dense fine-grained structure with grain sizes of the order of 16-30 nm. [0103] Determination of the phase composition of silicon carbide fibers was carried out by powder x-ray diffraction, on a Broker D8 Advance Vario powder x-ray diffractometer equipped with a Ge (111) monochromator (CuKal radiation) and a LynxEye position-sensitive detector.

[0104] As a result of the conducted studies, it was found that all peaks in the diffraction pattern of the samples correspond to the silicon carbide phase. The strong broadening of the peaks is indicative of the low size of the coherent scattering regions. The particle size, estimated by the Williamson-Hall method, is 1.4 - 2.0 nm.

[0105] The determination of the content of oxygen, silicon, carbon, as well as other possible impurities in the 0-SiC fiber, was carried out using an SEM with an EMF attachment (energy dispersive spectrum). As a result of the studies, it was found that -SiC fibers consist of silicon (43-56 at.%), carbon (31-46 at.%) and oxygen (8-11 at.%). Foreign impurities in the fibers and on their surface were not found.

[0106] Studies have also been conducted on the mechanical properties of [3-SiC fibers.

[0107] To determine the average diameter of a silicon carbide fiber, a metallographic microvisor p-Vizo-MET-221 Lomo Photonica (Russia) was used with a hardware magnification x200 - x500.

[0108] Physical-mechanical tests of the fibers were carried out by breaking a prototype fiber using an Instron 5942 tensile testing machine.

[0109] In the table in FIG. 32 (sample No. 1) shows the characteristics of the accumulated batch of fiber [3-SiC, and in Fig. 33-34 show samples of a batch of silicon carbide fibers.

[IT] Example 2. [0111] The formation of a multifilament polymer fiber, which is the first stage of the method for producing coreless silicon carbide fiber P-modification, was carried out by extrusion from polycarbosilane (PCS).

[0112] Preliminary, the optimal processing temperatures were determined for the PKC sample. As a result, data were obtained: softening temperature (Ti) 170°C, fiberization start temperature (Tg) 220°C and dropping point (Tz) 280°C. The temperature of a good thread T _{x n} . (optimum molding temperature) was chosen 30°C more than the fiberization start temperature - 250°C. In the table in Fig. 3 shows the temperature characteristics, sample No. 2.

[0113] Next, the extruder (extrusion-forming unit) was turned on in advance and the heating of the working parts was set up, taking into account the optimal molding temperature (T _{x n} .) 250°C.

[0114] A schematic diagram of an extruder for forming a multifilament polymer fiber from PCS is shown in FIG. 4, the scheme of the spinneret set 9 is shown in FIG. 5, they are described in example 7. The number of holes in the die 14 used was 48 pcs. The working volume of the gear pump 7 is 0.6 cm ³ .

[0115] PKS was taken in the form of pieces and loaded into hopper 1 (see Fig. 4), purged with an inert gas to prevent the interaction of PKS with atmospheric oxygen, while hopper 1 was evacuated using a vacuum pump 12.

[0116] The polymeric material was fed from the hopper 1 to the polymer melt preparation unit, heated by the heating elements 6 to a temperature of 250°C. The polymer melt preparation unit consists of a material cylinder 2 and a screw 3, which is driven through a clutch 4 by an electric drive of the extruder 5.

[0117] As a result of heating, the PCS pieces and chips were transformed into a PCS melt with a viscosity of 141.0 Pa-s.

[0118] Then, the prepared PKC melt was fed to a sealed heated fiber spinning unit, in which the gear pump 7 with an adjustable speed, driven by the electric motor of the gear pump drive 8, continuously at a speed of 12 rpm and a pressure of 6 MPa pumped the melted PKC onto spinneret set 9. At the same time, the PCS melt, being distributed over the entire surface area of the spinneret 14, was forced through all 48 holes at the same speed at a pressure of 1.84 MPa, which led to the formation of a bundle of continuous fibers.

[0119] Next, gradual cooling was carried out, followed by curing of the resulting bundle of fibers in an inert atmosphere - argon gas. Cooling first occurs after exiting the spunbond kit 9 in a heated under-spinning furnace 10 in an inert gas environment, then in an inert gas environment outside the under-spinning furnace 10 to room temperature, while the cooled formed polymer fiber was received at the fiber winding device 11.

[0120] The winding speed in the fiber winder is 798 rpm, which is faster than the 12 rpm feed rate of the molten polymer fiber from the spinneret, for under-die drawing of the fiber.

[0121] The spreader speed in the fiber spreader, equal to 33 rpm, allowed to change the angle of winding the fiber onto the spool. The receiving device, which includes a fiber winder and a fiber spreader, is shown in FIG. 7.

[0122] The PCS samples showed excellent formability, were drawn into a thin continuous filament, extrusion proceeded without pressure surges, and no gas inclusions were observed in the extrudate.

[0123] As a result, a batch of PCS polymer fibers was produced, which are brittle white fibers with a thickness of 25 microns.

[0124] After spinning, the fibers wound on the graphite packages were weighed on an analytical balance.

[0125] In the table in FIG. 10 shows the main parameters of fiber extrusion on the extrusion-forming unit, sample No. 2.

[0126] Next, the second stage of the method was carried out, namely, the polymer fiber was subjected to thermochemical crosslinking - curing (oxidation) in air at a temperature of 200 ° C with a heating rate of 6 ° C / h.

[0127] The crosslinking of polymer molecules occurred due to the oxidation of the Si-H and Si-CP3 groups and the formation of new Si-0-Si and Si-0-C bonds.

[0128] The fibers acquired a yellow tint, while their weight increased by an average of 10%.

[0129] After the oxidation process, the fibers wound on graphite packages were weighed on an analytical balance and installed on a special creel made of graphite. In FIG. 11 shows a graphite creel.

[0130] For the implementation of the third stage of the method, the cured PCS polymer fiber was transferred to the ceramic phase, that is, the sample was carbonized. To do this, it was subjected to processing in a high-temperature vacuum furnace with stepped heating up to 1100-1300 ° C in an inert atmosphere (nitrogen gases N2 and Ar). [0131] The fiber creel was installed in the carbonization furnace as shown in FIG. 12, the oven was closed and evacuated. Next, the furnace was filled with an inert gas, and the process of PCS fiber carbonization was carried out with stepped heating according to a program that included the following stages:

- Vacuuming the oven (up to 100 Pa);

- Filling the furnace space with an inert gas;

- Heating up to 400 °C at a heating rate of 2.5 °C/min;

- Exposure at 400 °C (30 minutes);

- Heating up to 700 °C at a heating rate of 4 °C/min;

- Exposure at 700 °C (30 minutes);

- Heating up to 1250 °C at a heating rate of 2.5 °C/min;

- Exposure at 1250 °C (30 minutes);

- Furnace cooling.

[0132] This program is shown in FIG. 2.

[0133] Oxidized fiber TGA data supporting the choice of program temperature parameters is shown in FIG. 13.

[0134] Photographs of the α-SiC fiber after the carbonization process are shown in FIG. 14-19.

[0135] The resulting P-modification silicon carbide fibers had a thickness of 15-22 microns. After carbonization, the fibers acquired a black color, a shiny surface, significant shrinkage (up to 25%) became noticeable, color shades of blue and brown were observed.

[0136] The process of obtaining silicon carbide fibers in the fourth, final stage includes sizing. A solution of polyvinyl alcohol with distilled water with a concentration of 0.25% was taken as a coupling agent.

[0137] The application of the sizing was carried out in accordance with the method shown in FIG. 20. Description of the method is given in example 7.

[0138] As a result, more than 100 meters of continuous length of finished sized multifilament 0-SiC was obtained.

[0139] The manufactured P-SiC fiber was sent for physical and chemical studies using optical microscopy, SEM, TGA, XRF, etc.

[0140] The studies carried out are similar to those described in Example 7. The results obtained are shown in FIG. 22-23, which shows the surface of the -SiC fiber (x500 magnification) - the fibers are smooth, without defects and have a fine-grained structure. [0141] SEM surface morphology data for 0-SiC fibers are shown in Figures 24-31. SEM micrographs showed that p-SiC ceramic fibers with a diameter of 15–22 μm have a dense fine-grained structure with grain sizes of the order of 16–30 nm.

[0142] As a result of the studies on the phase composition, it was found that all the peaks in the diffraction pattern of the samples correspond to the silicon carbide phase. The strong broadening of the peaks is indicative of the low size of the coherent scattering regions. The particle size, estimated by the Williamson-Hall method, is 1.4 - 2.0 nm.

[0143] The determination of the content of oxygen, silicon, carbon, as well as other possible impurities in the -SiC fiber, was carried out using an SEM with an EMF attachment. As a result of the studies, it was found that p-SiC fibers consist of silicon (43-56 at.%), carbon (31-46 at.%) and oxygen (8-11 at.%). Foreign impurities in the fibers and on their surface were not found.

[0144] Studies have also been conducted on the mechanical properties of the -SiC fiber. In the table in Fig. 32 (Sample No. 2) shows the characteristics of the produced batch of P-SiC fiber, and FIG. 33-34 show samples of a batch of silicon carbide fibers. [0145]

[0146] Example 3.

[0147] The formation of a multifilament polymer fiber, which is the first step in the process for producing a coreless silicon carbide fiber. P-modification was carried out by extrusion from polycarbosilane (PCS).

[0148] Previously, the optimal processing temperatures were determined for the PKC sample. As a result, data were obtained: softening temperature (Ti) 180°C, fiberization start temperature (Tg) 200°C and dropping point (Tz) 290°C. The temperature of a good thread T _{x n} . (optimum molding temperature) was chosen 40°C more than the fiberization start temperature - 240°C. In the table in Fig. 3 shows the temperature characteristics, sample No. 3.

[0149] Next, the extruder (extrusion-forming unit) was turned on in advance and the heating of the working parts was set up, taking into account the optimal molding temperature (Tkh.n.) 240°C.

[0150] A schematic diagram of an extruder for forming a multifilament polymer fiber from PCS is shown in FIG. 4, the scheme of the spinneret set 9 is shown in FIG. 5, they are described in example 7. The number of holes in the die 14 used was 48 pcs. The working volume of the gear pump 7 is 0.6 cm ³ . [0151] PKS was taken in the form of chips with a size of 2 - 7 mm and loaded into hopper 1 (see Fig. 4), it was purged with an inert gas to prevent the interaction of PKS with atmospheric oxygen, while hopper 1 was evacuated using a vacuum pump 12.

[0152] The polymeric material was fed from the hopper 1 to the polymer melt preparation unit, heated by the heating elements 6 to a temperature of 240°C. The polymer melt preparation unit consists of a material cylinder 2 and a screw 3, which is driven through a clutch 4 by an electric drive of the extruder 5.

[0153] As a result of heating, the PCS was transformed into a PCS melt with a viscosity of 919.9 Pa-s.

[0154] Then, the prepared PKS melt was fed to a sealed heated fiber spinning unit, in which the gear pump 7 with adjustable speed, driven by the electric motor of the gear pump drive 8, continuously at a speed of 4 rpm and a pressure of 6 MPa pumped the melted PKS onto spinneret set 9. At the same time, the PCS melt, being distributed over the entire surface area of the spinneret 14, was forced through all 48 holes at the same speed at a pressure of 4.00 MPa, which led to the formation of a bundle of continuous fibers.

[0155] Next, gradual cooling was carried out, followed by curing of the resulting bundle of fibers in an inert atmosphere - nitrogen gas. First, cooling occurs after exiting the spunbond set 9 in a heated under-spinning furnace 10 in an inert gas environment, then in an inert gas environment outside the under-spinning furnace 10 to room temperature, while the cooled formed polymer fiber was received at the fiber winding device 11.

[0156] The winding speed in the fiber winder is 550 rpm, which is a higher speed than the feed rate of the molten polymer fiber from the spinneret - 4 rpm, for under-die drawing of the fiber.

[0157] The spreader speed in the fiber spreader, equal to 30 rpm, made it possible to change the angle of winding the fiber onto the spool. The receiving device, which includes a fiber winder and a fiber spreader, is shown in FIG. 7.

[0158] The PCS samples showed excellent formability, were drawn into a thin continuous thread, the extrusion was without pressure surges, and no gas inclusions were observed in the extrudate.

[0159] As a result, a batch of PCS polymer fibers was produced, which are brittle white fibers with a thickness of 25 microns. After spinning, the fibers wound on graphite packages were weighed on an analytical balance. [0160] In the table in FIG. 10 shows the main parameters of fiber extrusion on the extrusion-forming unit, sample No. 3.

[0161] Next, the second stage of the method was carried out, namely, the polymer fiber was subjected to thermochemical crosslinking - curing (oxidation) in air at a temperature of 250 ° C with a heating rate of 10 ° C / h.

[0162] The crosslinking of polymer molecules occurred due to the oxidation of the Si-H and Si-CH3 groups and the formation of new Si-0-Si and Si-O-C bonds.

[0163] The fibers acquired a yellow tint, while their weight increased by an average of 15%.

[0164] After the oxidation process, the fibers wound on graphite packages were weighed on an analytical balance and installed on a special creel made of graphite. In FIG. 11 shows a graphite creel.

[0165] For the implementation of the third stage of the method, the cured PCS polymer fiber was transferred to the ceramic phase, that is, the sample was carbonized. To do this, it was subjected to processing in a high-temperature vacuum furnace with stepped heating up to 1100-1300 ° C in an inert atmosphere (nitrogen gases N2 and Ar).

[0166] The fiber creel was installed in the carbonization furnace as shown in FIG. 12, the oven was closed and evacuated. Next, the process of PCS-fiber carbonization was carried out in a vacuum with stepwise heating according to a program that included the following stages:

- Vacuuming the oven (up to 100 Pa);

- Filling the furnace space with an inert gas;

- Heating up to 400 °C at a heating rate of 5 °C/min;

- Exposure at 400 °C (30 minutes);

- Heating up to 700 °C at a heating rate of 5 °C/min;

- Exposure at 700 °C (30 minutes);

- Heating up to 1250 °C at a heating rate of 5 °C/min;

- Exposure at 1250 °C (30 minutes);

- Furnace cooling.

[0167] This program is shown in FIG. 2.

[0168] Oxidized fiber TGA data supporting the selection of program temperature parameters is shown in FIG. 13.

[0169] Photographs of the 0-SiC fiber after the carbonization process are shown in FIG. 14-19. [0170] The formed P-modification silicon carbide fibers had a thickness of 15-22 microns. After carbonization, the fibers acquired a black color, a shiny surface, significant shrinkage (up to 25%) became noticeable, color shades of blue and brown were observed.

[0171] The process of obtaining silicon carbide fibers in the fourth, final stage includes sizing. A solution of polyvinyl alcohol with distilled water with a concentration of 0.25% was taken as a coupling agent.

[0172] The application of the sizing was carried out in accordance with the method shown in FIG. 20. Description of the method is given in example 7.

[0173] As a result, more than 100 meters of continuous length of finished sized multifilament 0-SiC was obtained.

[0174] The manufactured P-SiC fiber was sent for physical and chemical studies using optical microscopy, SEM, TGA, XRF, etc.

[0175] The studies carried out are similar to those described in Example 1. The results obtained are shown in FIG. 22-23, which shows the surface of the p-SiC fiber (x500 magnification) - the fibers are smooth, without defects and have a fine-grained structure.

[0176] Data on the surface morphology of the P-SiC fiber by SEM are presented in figures 24-31. SEM micrographs showed that α-SiC ceramic fibers with a diameter of 15-22 μm have a dense fine-grained structure with grain sizes of the order of 16-30 nm.

[0177] As a result of the studies on the phase composition, it was found that all the peaks in the diffraction pattern of the samples correspond to the silicon carbide phase. The strong broadening of the peaks is indicative of the low size of the coherent scattering regions. The particle size, estimated by the Williamson-Hall method, is 1.4 - 2.0 nm.

[0178] The determination of the content of oxygen, silicon, carbon, as well as other possible impurities in the 0-SiC fiber, was carried out using an SEM with an EMF attachment. As a result of the studies, it was found that P-SiC fibers consist of silicon (43-56 at.%), carbon (31-46 at.%) and oxygen (8-11 at.%). Foreign impurities in the fibers and on their surface were not found.

[0179] Studies have also been conducted on the mechanical properties of the P-SiC fiber. In the table in Fig. 32 (Sample No. 3) shows the characteristics of the produced batch of P-SiC fiber, and FIG. 33-34 show samples of a batch of silicon carbide fibers. [0180] Thus, the invention provides: the possibility of obtaining continuous coreless p-modification silicon carbide fibers, - increasing their strength characteristics, in particular, increasing tensile strength, achieved through optimally selected parameters for molding,

- increased resistance to high temperatures and high creep resistance, in particular due to the oxygen content,

- stability of fiber properties,

- the optimal average fiber diameter is 15-22 microns,

- simplification of the procedure for preparing the used carbon fiber precursor, - the absence of impurities in the composition of the fibers, namely, the fibers consist of silicon 43-56 at. % and carbon 31–46 at. %, and also have a low oxygen content - 8-11 at. %.

Claims

Formula

1. A method for producing a coreless silicon carbide fiber - modification, including successive stages, according to which: at the first stage, the temperature of the beginning of fiber formation and the dropping point of each batch of polycarbosilane are preliminarily determined, and then a multifilament polymer fiber is formed by extrusion from a melt of polycarbosilane, for which polycarbosilane is introduced into extruder at room temperature, the polycarbosilane is heated to the optimal molding temperature, selected from the range above the fiber formation start temperature and below the dropping point, to obtain its homogeneous melt, the polycarbosilane melt is squeezed out through the spinneret at a pressure of 1.5-4.0 MPa to form a polymer fiber , gradually cool it in an inert atmosphere to room temperature, followed by curing, while accepting the polymer fiber to the fiber winding device, which has a winding speed greater than the possibility of supplying a polymer fiber, with the formation of a shaped polymer fiber, at the second stage, thermal-oxidative crosslinking is carried out, for which the resulting shaped polymer fibers wound on graphite packages are cured in an oxidation furnace at a temperature of 175-250 ° C with a heating rate of 3-10 ° C / h until the mass increases by 6-15%, at the third stage, the obtained cured polymer fiber is carbonized with its transfer to the ceramic phase, for which the cured polymer fiber is processed in a high-temperature furnace in an inert atmosphere or in vacuum with stepped heating to a temperature of 1100-1300 °C at a heating rate of 2.5-5 °C/min, while exposure is carried out for 30 minutes at temperatures of 400, 700 and 1250 °C, and then the high-temperature furnace is cooled, at the fourth stage, the resulting p-modification silicon carbide fiber is finished.

2. The method according to p. 1, characterized in that polycarbosilane is synthesized from polydimethylsilane.

3. The method according to p. 1, characterized in that at the first stage polycarbosilane is introduced into the extruder in the form of pieces or granules or chips with a size of 2-7 mm.

23

4. The method according to p. 1, characterized in that at the first stage, the polycarbosilane is heated to the optimal molding temperature, selected from the interval above the fiberization start temperature by 30 + 40 °C.

5. The method according to p. 1, characterized in that nitrogen gas is used as an inert atmosphere.

6. The method according to p. 1, characterized in that argon gas is used as an inert atmosphere.

7. The method according to p. 1, characterized in that in the fourth stage, a solution of polyvinyl alcohol with a concentration of 0.25% is used as a coupling agent.

8. The method according to claim 7, characterized in that to remove the sizing, the coreless silicon carbide fiber of the 0-modification is washed in water having a temperature of 35 + 40 ° C and dried at a temperature of 600 ° C for 30 minutes in air.